US3073758A

US3073758A - Polarographic method and apparatus

Info

Publication number: US3073758A
Application number: US838131A
Authority: US
Inventors: Rolf K Ladisch
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-09-04
Filing date: 1959-09-04
Publication date: 1963-01-15
Anticipated expiration: 1980-01-15

Description

POLAROGRAPHIC METHOD AND APPARATUS ,POM KHK/ 770/504 ATTORNEY United States Patent tice 3,013,758 Patented Jan. 1963 3 073,758 POLAROGRAPHIC lt/ETHOD AND APPARATUS Rolf K. Ladisch, 255 Windermere, Lansdowne, Pa. Filed Sept. 4, 1959, Ser. No. 838,131 15 Claims. (Cl. 204-1) The present invention relates to an electrochemical method and apparatus in the iield of polarographic analysis. Characteristics as well as potentialities and li-mitations of polarography, derivative polarography, differential polarography, oscilloscopio polarography, controlled potential separations, and amperometry have been eX- tensively discussed in .the literature. See for instance Chemische Analysen mit dem Polarographen, by H. Hohn, l. Springer Verlag, Berlin, 1937; Polarography, by I. M. Kolthofr` and J. I. Lingane, lnterscience Publishers, New York, 1952; Die Polarographie in der Medizin, Biochemie und Pharmazie, by M. Brezina and P. Zuman, Akad. Verlagsges., Leipzig, 1956.

These methods are based on the fact that electrolysis of a solution between a polarizable electrode and a nonpolarizable electrode produces current-voltage relationships that are typical for the quantity and quality of the investigated test species in the solution. The outstanding feature of the classic polarographic electrolysis cell is the polarizable dropping mercury electrode, i.e., mercury droplets being discharged periodically into a solution from a fine bore capillary under a driving head of mercury. This very feature, which has permitted the initiation of eX- tremely useful polarographic methods in research Work, seems now to militate against a more general use of polarography as a common analytical method and particularly as a tool for monitoring and controlling industrial process streams. The periodic growth and fall of the mercury droplets cause oscillations in the current-voltage curves which are an obstacle in evaluating such curves. Also an undesired condenser current builds up whenever a new mercury droplet is being formed at the capillary. Furthermore, the size of the droplets is necessarily small, thus limiting the method to a fraction of the sensitivity that could otherwise be obtained. In addition, formation of these tiny droplets is a delicate process and is therefore affected by a number of incidental factors, such as mechanical vibrations,"slant of capillary, pulsation of test solution into the mouth of the capillary between drops, etc. It should be remembered in this connection that the reproducibility of the droplets with regard to their drop time and mass of mercury per drop must be practically perfect at all times to permit proper evaluation of the polarogram.

Many attempts have been made to substitute better means for the dropping mercury electrode, such as multicapillaries, ejection of a stream of mercury from the capillary into the test solution, bent electrodes to increase drop size, forced dislodgment of droplets from the capillary in rapid succession, synchronization of mercury droplets being dropped from pairs of capillaries, oscilloscopio sweeps over successive drops at definite recurring time intervals, etc. Some of these methods are cumbersome to practice, some are extremely complicated if not impracticable, and most have not produced more reliable data than the original polarographic method, although in cases of special application, some benefit has been derived from these suggestions.

The present invention employs simple polarizable mercury electrodes in lieu of the classic dropping mercury electrode. Essentially, the electrodes of this invention consist of a small amount of lmercury that is separated from but electrolytically is put in communication with the test solution by means of a thin porous wall of small dimensions. Preferably the mercury is held against the porous wall under light pressure. In this arrangement, the test solution under investigation migrates through the porous wall to be electro-deposited on the mercury.

The solid matrix of the porous wall must be chemically inert to both the mercury and the test solution under investigation. It must not conduct electric current. The structure of said wall should be as open as possible, i.e., the pore volume should -be as great as is consistent with a sufficient structural rigidity to build and operate such an electrode. The pores in said wall should be `small enough to prevent the mercury from entering them, but should be large enough to permit the test solution to diffuse freely through said porous wall to the mercury. In general, porous media with controlled porosities which are available in commerce, such as microporous ceramic material with a pore volume of about 40% and an average pore diameter of about 5 to l0 microns, or fritted sintered glass with a similar degree of porosity and a similar pore size, are preferred in connection with pressures on the mercury ranging from about mm. Hg to 200 mm. Hg. The porous wall should have a thin cross section to facilitate the migration of the test solution to and from the mercury; it should be just thick enough to have sufficient structural strength for constructing and handling the electrode. The above mentioned microporous ceramic media may be formed into objects of the desired shapes with wall thicknesses of the order of one-sixteenth of an inch or even less.

The mercury in this electrode is preferably held against the porous wall under slightpressure either derived from an elevated supply of mercury connected to it, or from a pump driving the mercury through the electrode along the porous Wall, or by any other suitable means. This pressure aids greatly in maintaining a coherent mercury surface at the boundary between the mercury and the porous wall, and thus it substantially eliminates migration of the test solution beyond the porous wall into that portion of the electrode that is assigned to the mercury. n

The mercury may be kept stationary Within the electrode, provided it is frequently renewed and provided the duration of the test runs is limited to short periods, so as not to contaminate the mercury appreciably with deposits of the electrode reaction. However, flow or streaming of the mercury through the electrode along the porous wall is preferred, particularly in apparatus designed for continuous analysis, such as for process streams of the chemical industry. The ow of the mercury facilitates a continuous renewal of the mercury surface in the region where the electrode process takes place.

The test solution contacting the mercury in the electrode via the porous wall may be stationary since the rate of migration of the test species to the mercury is generally greater than the depletion of the test species in the solution. The depletion is generally minimal, because very low current densities of the order of a few microamperes are normally used to deposit the test species on the mercury. However, agitation and/or flow of the test soluvtion along the porous wall does not interfere ywith the analysis as would be the case with most polarographic cells employed heretofore. This feature is particularly useful in equipment facilitating the continuous analysis of an industrial process stream or the like. i

The polarizable electrode of the present invention offers a number of advantages over the conventional dropping mercury electrodes, among which are simplicity of design and operation, increased sensitivity, increased resolution of waves belonging to substances With'a similar [decomposition potential, substantial elimination of the condenser current, greatly increased -readability of polarograms due to elimination of oscillations in the. curves, minimization of polarographic maxima, and in 3 general ruggedness, since the electrode of the invention is neither sensitive to mechanical vibrations, nor to agitation of the test solution during analysis, nor to small changes in the height of the driving head of mercury.

In the accompanying drawings, two embodiments of the invention are shown by way of illustration. In a companion applicaton tiled lune 22, 1959, Serial No. 821,978, I have shown two additional polarographic cells, and have presented broad claims which are intended to cover the four cells of the two applications, as well as the method which is part of my invention.

In said drawings,

FIG. 1 is a somewhat diagrammatic vertical section through a polarographic cell which is particularly adapted for the simultaneous determination of two test specimens in the same solution; v

FIG. 2 is a similar view of another form of polarographic cell which is particularly useful for controlled potential separations.

Referring particularly to fthe drawings and iirst to FiG. 1, there is shown a polarographic cell including a mercury reservoir 11, a stand tube 12 leading oilC from the bottom of reservoir 11, and a supply of mercury 13 in reservoir 11 and stand tube 12. A second mercury reservoir 11B, stand tube 12a, and mercury supply 13a are shown alongside of the

corresponding parts

11, 12 and 13. The lower ends of the stand tubes are bent to extend substantially horizontally, with the lower end of stand tube 12a directly below the discharge end of stand tube 12. The extremities 20, 2011 of lthe stand tubes are iixed to the lower portion of a vessel 32. The vessel 32 may be a generally cylindrical body of glass or a resinous plastic such as epoxy resin, or any other suitable material, and it provides a container for the test solution under investigation, which is poured in at the top of the vessel. A central bore 30 extends through the relatively massive bottom of vessel 32 and in this bore a microporous ceramic rod 34 is cemented, so that its upper end is in contact with the test solution 33. Microporous rod 34 may have a diameter of about onefourth of an inch, and an average pore diameter of about to 15 microns. The lower end of rod 34 extends into an open container or beaker 13, in the bottom of which is a mercury pool electrode 19. Lying on top of electrode 19 is an appropriate electrolyte, such as an aliquot of the test solution 33. The microporous rod must be of such material that it is inert to mercury and the test solution, and does not conduct electricity. With pores of the size range mentioned above, the mercury cannot enter the pores of rod 34 but the test solution may migrate freely through the pores.

Small mercury-conducting

passages

36, 36a run diametrically through the bottom of vessel 32 and registering passages 35, 35a are provided in the microporous rod 34. Connection tubes 37, 37a are iitted in the bottom of vessel 32 in alignment with

passages

36, 36a respectively, and are sealed in position by rubber O-rings 3S, 38a. A sleeve 39 of rubber tubing couples tubes 37, 37a together. The common outlet tube 40 for tubes 37, 37a extends to a manually adjusted needle valve 41 having a discharge tube or outlet 42 for the mercury. A beaker or other vessel (not shown) will be placed be- 10W outlet 42 to collect the mercury dropping or streaming therefrom. Mercury supplies 13 and 1,3a and mercury pool 19 are connected by

electrical conductors

58, 59 to apparatus (not shown) for determining of currentvoltage data in the manner well known in the art.

In operation, mercury from the supplies 13, 13a flows through the respective stand tubes 12, 12a, through

passages

36, 36a, 35, 35-, tubes 37, 37a and through ther needle valve 41 at a rate determined by `the opening of the needle valve. For example, the velocity of flow of the mercury from the outlet 42 may be approximately one-half of one gram per minute. Test solution 33 slowly ows by gravity through microporous rod 34 75 into vessel 18 forming electrolyte pool 33 therein. Electro-deposition of the test solution takes place at the mercury surface formed by the mercury flowing through passages 35, 3521 in rod 34. The mercury in each of the two reservoirs 11, 11a may be connected to a predetermined applied potential, each in the manner well known in the art, for determining single desired species in the test solution. Thus the test solution may be analyzed simultaneously for two species of substance in the same test solution, by polarographic methods, when using the apparatus of FIG. l.

It is obvious that simultaneous analysis of several more sepcies of test substance in one test solution may be accomplished, if so desired, by constructing a cell assembly of the type shown in FIG. l, but having additional passages through the microporous rod, means for ilowing mercury through such passages, and needle valve or other means for a nice control of such ilow.

Now referring to the embodiment of the invention shown in FIG. 2, a mercury reservoir 11 has a stand tube 12 leading olf from its bottom wall and holds a supply of mercury 13. The discharge end 20 of the stand tube is sealed in the relatively massive bottom portion 45 of a vessel 5t) which holds the test solution 51, poured in at the top. A small transverse bore 52 is provided in said bottom portion 45 to receive the mercury ilowing from the stand tube and conduct it to a sm-all chamber S3 having cylindrical walls, located centrally (axially) of the bottom portion 45.

A microporous rod 54 is secured in said bottom portion to extend axially thereof, the upper end of the microporous rod being in contact with the test solution 51. Rod 54 extends axially through chamber 53 and projects downwardly from the bottom portion 45, its lower end being within an open container or beaker 18. A mercury -pool electrode 19 is in the bottom of beaker 18, and lying on top of the electrode 19 is an appropriate electrolyte, such as an aliquot of the test solution 51. Rod 54 is a ceramic solid rod about one-fourth of an inch in diameter, with an average pore diameter of about l() to 15 microns. Its material is such that it is inert to mercury and to the test solution and it does not conduct electricity. With pores of the size range mentioned, Ithe mercury in chamber 53 can not enter the pores, but the test solution may migrate freely through the pores and ows down the rod by gravity from the vessel S0 to beaker 18.

As stated above, the transverse bore 52 delivers mercury to the central chamber 53 at the bottom thereof. Another small transverse bore 55 is provided in bottom portion 4S -at the -top of chamber 53 to conduct the mercury away. A tube 56 receives the mercury from bore 5S and conducts it to a needle valve 41, thence to a discharge tube or outlet 4Z. A beaker or other vessel (not shown) will be placed below outlet 42 to collect the mercury dropping or streaming therefrom. The llow of the mercury will obviously be governed by the degree of opening of the needle valve. The velocity of ilow of the mercury may be adjusted by the needle valve to approximately one-half of one gram per minute. The mercury supply 13 and the mercury pool electrode 19 are connected by suitable conductors 6i), 61 to apparatus for determining of current-voltage data in the manner well Aknown in the art of polarography.

In operation, as the test solution 51 slowly descends microporous rod 54, the test solution is plated out with respect to the desired test species, this test specimen being deposited on the mercury in chamber S3 adjacent rod 54. The current derived from this unit is a direct measure of the concentration of the test species to be analyzed. The' embodiment of FIG. 2 permits one to plate out the desired test `species completely, which process is well known in the art as controlled potential separation. In continuous analysis, such as for instance with industrial process streams, any change of concentraaorsfrss tion of .the investigated species in the test solution will be indicated by a change in the current derived from this cell. Due to the relatively large surface area of the mercury in chamber 53 and `therefore the relatively large currents obtained from even minute quantities of test species, the form of FIG. 2 is particularly valuable in the analysis of trace materials.

1t will be obvious to those skilled in the art that the cells shown in FIGS. l and 2 may be combined to form an apparatus that would be very useful for the simultaneous separation and determination of complex test solutions in a single operation combining controlled potential separation and polarographic current-voltage analysis. Other changes in the apparatus will be obvious to those skilled in the art of polarography.

Having described two of the many forms which my invention may take, what I claim as new and desire to secure by Letters Patent is:

1. Electrochemical apparatus comprising, in combination, a vessel having a chamber adapted to contain a quantity of a test solution under investigation; a microporous rod whose upper end is adapted to be in contact with the test solution in said chamber; the lower end of said rod being below the bottom of the chamber; said rod being of a material that is inert to mercury and to the test solution and that is non-conductive of electricity; a source of mercury; means conducting the mercury under low pressure to a wall of said rod which is also adapted to be contacted by said test solution; the pores of said rod being large enough and numerous enough to permit gravity ow of the test solution from said chamber length- Wise of said rod, but said pores precluding the mercury from entering them; means for conducting the mercury away from said rod wall so that there may be a continuous ow of mercury past said rod wall; a suitable electrode adapted to be in electrolytic contact with said test solution below the lower end of said rod; and means for applying an electrical potential to said mercury versus said electrode, so as to produce a desired electrode reaction between the test solution andthe mercury near said rod wall.

2. The invention dened in claim 1, wherein the microporous rod is a solid ceramic rod about one-quarter of an inch in diameter having an average pore diameter of about l0 to 15 microns.

3. The invention defined in claim 1, wherein the means for conducting mercury away from said rod is coupled to a needle valve which is manually controllable to regulate ilow of mercury past said wall of said rod.

4. The invention defined in claim l, wherein there is a container surrounding the lower end of said microporous rod, a mercury pool electrode being in the bottom of said container, and an appropriate electrolyte lying on top of said mercury pool electrode and contacting the lower end of said rod.

5. The invention defined in claim 1, wherein the microporous rod has a transverse passage therethrough; ther means for conducting mercury to said rod -discharges the mercury into said transverse passage; and the means for conducting mercury away from said rod receives the mercury that has owed through said transverse passage.

6. The invention defined in claim 5, wherein there is a manually controlled needle valve to which the mercury is led by said means for conducting mercury away from said rod.

7. The invention dened in claim 1, wherein there are a plurality of separate sources of mercury, a plurality of transverse passages in the microporous rod, a plurality of separate means for conducting mercury under low pressure to the respective transverse passages, and means for conducting away the mercury that has traversed said transverse passages.

8. The invention dened in claim 7, wherein there is a manually controlled needle valve coupled to said means for conducting away the mercury that has traversed said transverse passages.

9. The invention defined in claim 7, wherein the separate sources of mercury are reservoirs which are elevated relative to the transverse passages in the microporous rod so as' to insure gravity tlow of the mercury through said passages.

l0. The invention defined in claim 1, wherein the vessel has a mercury chamber formed therein and surrounding the microporous rod and having no connection with the test solution chamber; the means for conducting mercury to said-rod discharges the mercury into said mercury chamber; and the means for conducting the mercury away from said rod conducts the mercury from said mercury chamber.

11. The invention defined in claim 10, wherein there is a manually conrtolled needle valve to which the mercury is conducted by said means for conducting mercury away from said rod. v

12. The invention defined in claim 10, wherein there is a container surrounding the lower end of said microporous rod; a mercury pool electrode being in the bottom of said container; and an appropriate electrolyte lying on top of said mercury pool electrode and contacting the lower end of said rod.

13. An electrochemical method characterized by the step of causing a test solution under investigation to flow slowly through the pores of a microporous body; said body being a non-conductor of electricity and being inert to mercury and to the test solution; the pores of said body being small enough to prevent the mercury from entering; simultaneously causing mercury to ow under slight pressure along a wall of said body which is contacted by said test solution, and applying an electrical potential to the mercury via aA suitable electrode which is adapted to be in electrolytic contact withthe test solution, so as to produce a desired electrode reaction between the test solution and the mercury.

14. An electrochemical method characterized by the step of causing a test solution under investigation to flow slowly through the pores of a microporous rod; the rod being a non-conductor of electricity and being inert to mercury and to the test solution; the pores of said rod being small enough to precludev the mercury from entering; simultaneously causing mercury to flow along a passage inside said rod; and applying an electrical potential to the mercury via a suitable electrode which is adapted to be in electrolytic contact with the test solution, so as to produce a desired electrode reaction between the test solution and the mercury owing along the passage inside said rod.

15. An electrochemical method characterized by the step of causing a test solution under investigation to flow slowly through the pores of a microporous rod; the rod being a non-conductor of electricity and being inert to mercury and to the test solution; the pores of the rod being small enough to preclude the mercury from entering; simultaneously causing mercury to ow around a portion of the exterior of said rod, and applying an electrical potential to themercury via a suitable electrode whichy is adapted to be in electrolytic contact with the test solution, so that the test solution is plated out with respect to the desired test species, the test specimen beingv electro-deposited on the mercury surrounding the rod.

y References Cited in the le of this patent UNITED STATES PATENTS

Claims

13. AN ELECTROCHEMICAL METHOD CHARACTERIZED BY THE STEP OF CAUSING A TEST SOLUTION UNDER INVESTIGATION TO FLOW SLOWLY THROUGH THE PORES OF A MICROPOROUS BODY; SAID BODY BEING A NON-CONDUCTOR OF ELECTIRCITY AND BEING PART TO MERCURY AND TO THE TEST SOLUTION; THE PORES OF SAID BODY BEING SMALL ENOUGH TO PREVENT THE MERCURY FROM ENTERING; SIMULTANEOUSLY CAUSING MERCURY TO FLOW UNDER SLIGHT PRESSURE ALONG A WALL OF SAID BODY WHICH IS CONTACTED BY SAID TEST SOLUTION, AND APPLYING AN ELECTRICAL POTENTIAL TO THE MERCURY VIA A SUITABLE ELECTORDE WHICH IS ADAPTED TO BE IN ELECTROLYTIC CONTACT WITH THE TEST SOLUTION, SO AS TO PRODUCE A DESIRED ELECTRODE REACTION BETWEEN THE TEST SOLUTION AND THE MERCURY.